How to add thin wispy clouds or fog to my blender 3d scene of building
Blender internal:
You could try the Cloud Generator addon. This is good for making 3D volumetric clouds, but note that it will only work for BI, as cycles does not yet have volumetric rendering.

Enable the Cloud generator addon in User preferences &gt; Addons. This addon is bundled with blender by default, so it should be in the list:


Create a bounding shape for your cloud, then press Generate cloud in 3D view &gt; Tool Shelf (T) &gt; Cloud generator with it selected:



This will create a point cloud for the cloud and setup the volume materials.
For more info take a look at this tutorial on blender cookie.
Cycles:
Because cycles does not yet have volumetrics, you will have to fake it.
Here are some options:

Render the clouds in BI and mix in the compositor

Use a transparent plane with an image texture (may look okay when seen from a distance)


Related questions:

Volumetric Shading in Cycles

Creating volumetric lighting (crepuscular rays) in Cycles

Rendering smoke in Cycles?